Rotary valve actuators can be double or single-acting devices. A double-acting rotary valve actuator is driven by fluid pressure entering a first chamber to move a piston and valve shaft in one rotational direction, or by fluid pressure entering a second chamber to move the piston and shaft in the opposite rotational direction. A single-acting rotary valve actuator is one having a spring mechanism to rotate the valve shaft in one direction, while fluid pressure, in the form of pressurized air, water, or hydraulic fluid, rotates the piston and valve shaft in the other direction.
Nearly all commercially available rotary valve actuators convert the linear motion of a piston or diaphragm to rotary motion of the valve by means of a lever, gear, or yoke mechanism. In these actuators the forces generated by the fluid pressures on the piston act through the mechanisms to cause rotary motion of the actuator and valve shafts. Reactions to the rotational forces cause side loading forces to act upon the piston cylinder sidewall and lever, gear or yoke mechanism. These side loading forces increase the friction between the components. This friction, plus the necessarily imperfect fit between moving components, cause a reduction in the actuator's efficiency, cycle life, and valve positioning accuracy.
One type of rotary valve actuator that utilizes direct rotary motion is known as a vane-type actuator. It uses a paddle mounted in an offset position on a shaft. Fluid pressures acting on the paddle move the paddle along an arcuate path and rotate the valve shaft. The vane-type rotary actuator, while simplistic, is noted for the great amount of leakage that occurs between the paddle housing and the paddle. Additionally, the vane-type rotary actuator could only be converted from a double-acting actuator to a single-acting actuator by employing a separate external spring-return device connected to the actuator.
Rotary actuators which utilize arcuate pistons to effect direct rotary movement of an output shaft provide an alternative to the linear-to-rotary type actuators and to the vane-type actuators. Unlike linear-to-rotary actuators, the rotary valve actuators which utilize arcuate pistons have no lost motion, have no side loading, and therefore have higher operating efficiencies, longer service life and improved positioning accuracy. Unlike vane-type actuators, arcuate piston type rotary actuators have practically no leakage problems and can achieve single action operation with a spring installed within the actuator housing.
The problems experienced with direct acting rotary valve actuators were partially overcome by Scobie as disclosed in U.S. Pat. No. 5,007,330, which issued Apr. 16, 1991. Scobie addressed the sealing problems known to occur with direct acting rotary valve actuators by utilizing an arcuate piston and by eliminating the use of a body housing that is split along the horizontal center axis. The housing of the actuator disclosed by Scobie consists of two sections that join to the side thereof. This housing construction improved the ability to achieve a tight seal between the housing sections as well as between the piston and the housing. However, Scobie employed a single-acting rotary valve actuator having a spring return mechanism that is mounted in an elongated tube that extends to the side of the housing, and which is quite cumbersome. His disclosure requires that a spring strap pass from the spring return mechanism through an opening in the actuator housing to the valve shaft. The requirement for an additional seal at this point is not eliminated. Further, the structure of the housing disclosed in the Scobie patent prohibits one from being able to economically change the type of output shaft, for example, to replace a female output shaft with a male output shaft. The construction of the housing will not enable the insertion of an integrally connected male shaft and piston assembly as there will not be room to maneuver the male shaft and piston assembly into the housing. An additional problem with changing output shafts is that either must be placed into an integral female shaft of a larger internal diameter. Therefore, them would be lost motion between the actuator components.
It is an object of this invention to provide a new and improved actuator housing that allows a complete piston assembly, including the arcuate piston, its output shaft and its lever arms to be inserted and removed from the housing.
It is a further object of this invention to provide a single-acting rotary valve actuator having an arcuate piston assembly that includes a spring means that can be wound to the desired tension before the piston assembly is mounted in the actuator housing and which spring means is combined with the arcuate piston assembly, and thereby causes no change in the external dimension or shape of the actuator.
Another object of the invention is to provide a new and improved rotary valve actuator where a spring having a variable amount of bias force holds a valve element in its normally-closed position, and where means including an arcuate piston is used to rotate the valve element to its open position.
Another object of the invention is to provide a new and improved rotary valve actuator having a spring loaded piston assembly which may be first inserted into its housing, after which one end of the spring may be secured to the housing of the actuator, such that the spring housing connection forces the piston assembly into a closed position within its housing.